I. To determine whether the addition of erlotinib (erlotinib hydrochloride) to gemcitabine (gemcitabine hydrochloride) adjuvant chemotherapy shows a signal for improved survival as compared to gemcitabine alone following R0 or R1 resection of head of pancreas adenocarcinoma (including adenocarcinoma of the head, neck, and uncinate process). (Phase II-R) II. To determine whether the use of concurrent fluoropyrimidine and radiotherapy following adjuvant gemcitabine based chemotherapy or non-gemcitabine based chemotherapy such as modified fluorouracil-leucovorin-irinotecan-oxaliplatin regimen (FOLFIRINOX) further enhances survival for such patients who are without evidence of progressive disease after 5 months of adjuvant chemotherapy. (Phase III)